1. Field of the Invention
The present invention relates to turntable devices employed for measuring angles of lock of a steerable wheel of an automobile.
2. Description of the Prior Art
A measuring device of this type has a horizontal turntable which is intended to receive a steerable wheel and is movably mounted on a stationary support so as to be capable both of free pivotal motion and of limited displacements in its own plane, the spindle of said turntable being coupled with an angular displacement pickup for measuring the angle of lock. The pickup is carried by an arm which is associated with the displaceable turntable and accompanies this latter in its movements of translation without being subjected to its pivotal movements. In order to determine the angles of lock of the two steerable wheels of an automobile, these wheels are accordingly placed on the turntables of two devices of this type, whereupon the desired measurements are made by turning the wheels successively in one direction and in the other. It is worthy of note in this connection that the possibility of limited displacements of each turntable in its own plane is necessary, taking into account the fact that the vertical axis of pivotal motion of each wheel is not usually located vertically above the axis of pivotal motion of the corresponding turntable.
In devices of this type which are at present in use, the turntable rests on the top surface of the stationary support by means of ball-bearings and its vertical spindle is engaged through an opening of said support. The cross-sectional area of this opening therefore determines the amplitude of the possible translational displacements of the turntable in its own plane.
However, it is possible that, while a wheel is being turned, the corresponding turntable moves in such a manner that its spindle is abuttingly applied against the edge of the opening of the corresponding support. This completely upsets the measurements made. The results thus obtained are completely erroneous. Furthermore, in current devices of this type, the operator has no possibility of detecting this malfunction other than by observing the absurd character of the measurements indicated by the corresponding device.
This defect is again found in the device disclosed in French patent No. 2,466,745 which describes a mechanical turntable of the type recalled in the foregoing and having a distinctive feature consisting of a system for taking up play. In fact, no means are provided in this device for preventing an erroneous measurement in the event that the top plate comes into a position of mechanical abutment on the bottom plate.
In British patent No. 2,106,254, there is described an apparatus for measuring side-slip or in other words a device for measuring the lateral displacement of an upper plate with respect to a lower plate which is rigidly fixed to the floor, this lateral displacement being caused by the passage of a vehicle wheel which is to be checked and exhibits toe-in or toe-out. A device of this type is in widespread use. Here again, however, an error of measurement cannot be avoided with this device if the upper plate is in a position of abutment.
For the reasons which have just been given, the aim of the invention is to improve the turntable devices of the type recalled earlier by equipping them with a system for avoiding errors of measurement of wheel-lock angles.